Hitherto, as a recording medium for digitally recording AV (Audio Visual) information such as an image and sound, an optical disc such as a CD (Compact Disc) or a DVD (Digital Versatile Disc) is generally used. In the optical disc, information corresponding to the AV information is recorded as a number of recording pits (also referred to as recording marks in a write-once or rewritable optical disc) having lengths in the circumferential direction corresponding to binary data of “0” or “1” on an information recording surface in a state where they are synchronized with the frequency of a preset recording clock signal.
Therefore, to optically accurately reproduce the AV information from such an optical disk, the timing (time relation) between a reproduction signal detected from an optical beam emitted to the optical disk and a reproduction clock signal generated on the optical disc player side as a reference of the reproducing process has to be the same as the timing (time relation) between a recording signal recorded on the optical disk (a recording signal corresponding to the AV information) and the recording clock signal used at the time of recording the recording signal on the optical disk. Since the recording signal and the recording clock signal have to be synchronized with each other as described above, the reproduction signal and the reproduction clock signal have to be also synchronized. To make the frequency of the reproduction clock signal synchronized with that of the reproduction signal, conventionally, the configuration of synchronizing the frequency of the reproduction clock signal with the frequency of the reproduction signal by using a so-called phase locked loop (PLL) circuit is used.
On the other hand, many of optical discs record AV information in a so-called CLV (Constant Line Velocity) method. However it is known that, in an optical disk on which AV information is recorded in the CLV method, in the case of retrieving information by moving an optical spot directly from the innermost radius side to the outermost radius side or directly from the outermost radius side to the outermost radius side, the difference between the frequency of a reproduction signal and the frequency of a reproduction clock signal principally becomes large immediately after the movement. When the difference increases, accurate reproducing process cannot be performed. With respect to the point, hitherto, as described in Japanese Unexamined Patent Application Publication No. H10-125010, by changing the rotation speed of a spindle motor (a spindle motor for rotating an optical disk itself) on the basis of expected linear velocity in a position on the optical disk after the retrieval, the difference between the frequency of the reproduction clock signal and the frequency of the reproduction signal is reduced.
However, generally, the response to a change in the rotation speed of the spindle motor is often slow. Consequently, in the case of retrieving information at high speed in the above-described range, even with the conventional method of predicting linear velocity in a position after the retrieval, the difference between the frequency of the reproduction clock signal and the frequency of the reproduction signal is still large.
In the case where the difference between the frequency of the reproduction signal and the frequency of the reproduction clock signal becomes large, the frequencies cannot be synchronized only by the conventional PLL circuit. Therefore, conventionally, the interval between synchronization signals (hereinafter, also properly referred to as “sync signals”) included in a reproduction signal or the length of the longest recording pit (hereinafter, the longest recording pit is also properly referred to as “longest pit”) is detected, the frequency of the reproduction clock signal is controlled by using the detected interval, length, or the like to reduce the difference between the frequency of the reproduction clock and the frequency of the reproduction signal to some extent and, after that, the sync control by the PLL circuit is performed.
In the case of controlling the frequency of the reproduction clock signal by using the length of the longest pit, more concretely, the error between the length of the longest pit determined as a standard and the length of the longest pit actually detected is detected, and the frequency of the reproduction clock signal is controlled so as to reduce the error. Hereinafter, the length of the longest pit will be generally referred to as “TMAX” and the error between TMAX determined as a standard and TMAX actually detected will be referred to as a “TMAX error”.